If you have Greenfoot 2.4, you can use the 'showText' method. If not, you can:
(1) use 'getBackground().drawString(String, int, int)'; or
(2) create a GreenfootImage using the GreenfootImage(String, int, Color, Color) method and either:
(a) use 'getBackground().drawImage(GreenfootImage, int, int)'; or,
(b) create an actor object and set the image to it and add it to the world
you must learn it yourself :D if you want to know something google it and then practise on your own and when it wont work after a while then you can ask the forum here :D thats the best way to learn it :D